1. Field of the Invention
The present invention relates to cables and, in particular, to optical fiber cables incorporating fibers which impart water-blocking and flame-retarding characteristics to the cables.
2. Description of the Related Art
Many products, such as telecommunications and power cables, for instance, typically are provided with one or more forms of water-blocking protection, i.e. waterproof outer jackets, water-blocking powders and yarns, etc. Generally, such water-blocking protection is intended to reduce or eliminate the possibility of water entering the product so as to reduce the potential for water damage or for impairment of transmission. For example, once water enters a telecommunications cable, such as through a splice closure of the cable, freezing of the water can induce microbending in the optical fibers of the cable, which can result in fiber degradation or increased signal loss of the cable.
Heretofore, some prior art cables have incorporated water-blocking material in order to prevent water penetration into and migration through the cables. An example of such a prior art cable is described in U.S. Pat. No. 5,390,273, issued to Rahman, for instance. In the cable described therein, water-blocking material is disposed in any otherwise empty spaces within a fluid impervious tube surrounding a plurality of optical fibers and between the barrier layer and the tube. The water-blocking and/or hydrogen absorbing compound can be in the form of a known type of grease and can be thixotropic so that there can be movement of the optical fibers with respect to each other and to tubes in the cable. Known types of water blocking compounds include a silicone based thixotropic compound, a compound based on hydrocarbon oils or polymeric liquids, such as polybutylene, and petroleum greases. The use of such a filling materials causes housekeeping problems, though, because these compositions of matter are somewhat messy to apply and require a cleaning agent such as a solvent to remove the filling compound to facilitate subsequent splicing. Also, care must be taken so that these agents do not affect adversely coloring material or coating material on the optical fiber. What is sought after is a cable in which the core is not filled with a grease-like material, but rather includes other provisions for blocking water flow along the core.
Heretofore, some prior art cables have also incorporated water-blocking tape in order to prevent water penetration into and migration through the cables. Examples of such prior art cables are described in U.S. Pat. No. 4,867,526, issued to Arroyo, for instance. In the cables described therein, water-blocking tape is disposed between the transmission media and the outer jacket of the cable, thereby forming a water-blocking barrier around the transmission media, i.e. optical fibers. The water-blocking tape incorporates super-absorbent materials, e.g. super-absorbent polymers (SAPs), which can absorb up to about one thousand (1000) times their weight in distilled water. The SAPs are provided in a loading of approximately 3.8-6.22 glft.sup.2. This process results in a tape that weighs approximately 10-80% more than the substrate of the tape. Upon contact with water, the SAPs of the water-blocking tape absorb the water and swell, thereby preventing migration of the water through the cable. Additionally, the swollen SAPs form a physical barrier which can prevent more water from entering the cable.
Other examples of prior art cables are described in U.S. Pat. No. 4,909,592, issued to Arroyo et al. These prior art cables incorporate a water blocking system within the core tube. The water blocking system comprises a tape or yarn or both or a plurality of tapes or yarns. The tapes or yarns are provided so as to intentionally cause the core to include voids, and do not completely fill the interstitial core area around the optical fibers disposed within the core tube. The interstitial voids allows the fibers to be substantially decoupled transversely from the sheath system of the cable. This allows substantial motion of the transmission medium in at least one transverse direction when the cable is stressed such as by bending of the cable. However, the creation of these voids may not effectively cushion and center the transmission medium within the core. Further, upon contact with water, the water blocking system may not swell enough so as to prevent water migration to the transmission medium.
Heretofore another prior art solution to attain the characteristics of both water-blocking and flame-returning has been to deposit the SAPs in the cable in powder form which has had some measure of success. However, there are problems with this method of water-blocking. First, the friction between the powder and the components of the cable causes high loss due to microbending. Second, because the powder is inserted into the cable via electrostatic deposition, the powder settles to the bottom of the cable over time. Third, the powder can be difficult to apply evenly and some of the powder is lost during assembly. Fourth, the powder cannot be used inside the core tube due to microbending loss. For the same reason, the water-blocking tapes described hereinabove exhibit a lot of the same problems as the powder form does, due to the use of long woven surfaces.
Therefore, there is a need for improved cables and fibers which address these and other shortcomings of the prior art.